Mitsuyoshi Fujiyama

Melting and crystallization behaviors of injection‐molded polypropylene

The melting and crystallization behaviors of the skin layer in an injection-molded isotactic polypropylene (PP) have been studied, mainly in comparison with those of the core layer and subsidiarily in comparison with those of a compression-molded PP and a nucleator (talc)–added PP. The skin layer contains about 5% crystals, which have a high melting point of up to 184°C. They thermally vanish by melting once. The subsequent melting history will scarcely affect the melting behaviors. On the other hand, crystallization behaviors are strongly affected by the melting history. The skin layer crystallizes in a wide temperature range at high temperature. This tendency weakens with increasing melting temperature, approaching a constant and that of the core layer above 230°C, which suggests that the memory effect of the residual structure of PP vanishes by melting above 230°C. In explaining these experimental results, it is assumed that the residual structure substance is a melt orientation of molecular chains that works as crystallization nuclei and that the vanishing of the residual structure is nothing but a relaxation of the melt orientation.

Structure and properties of injection moldings of polypropylene/polystyrene blends

A homoisotactic polypropylene (PP) was melt blended with 0–30 wt % of three kinds of polystyrene (PS) with melt flow indexes lower than, similar to, and higher than that of PP. The blends were injection molded at cylinder temperatures of 200–280°C, and the structure and properties of the injection moldings were studied. With PS blending, although the PP molding whitened, no surface defect such as layer peeling and pearl-like appearance occurred. The rigidity and dimensional accuracy of the molding improved without much deterioration in impact strength and heat resistance. At the same time the fluidity also improved. The injection moldings of PP/PS blends did not show clear skin/core structure under a polarizing microscope. The degrees of crystallinity and crystalline c-axis orientation decreased with PS blending. PS particles were the smallest when the ratio of the viscosity of the PS to that of PP at molding shear rate was slightly lower than unity.

Capillary flow properties of PVC melts (abstract)

The effects of degree of polymerization and temperature on flow properties of PVC melts have been studied by using a Koka Flow Tester. Each flow curve is expressed by two straight lines of different slopes which meet at a breaking point. The critical shear rate γwc’ at the breaking point increases as the degree of polymerization decreases or the temperature increases. The critical shear stress τc at γwc’ increases as the degree of polymerization decreases, being almost independent of the temperature. The end correction coefficient ν shows a minimum or begins to increase around γwc’. The coefficient ν increases as the degree of polymerization increases or the temperature decreases. The extrudate is smooth on the surface at low shear rates, shows a small wavy roughness (shark skin) at shear rates just below γwc’, becomes fairly smooth again beyond γwc’, and then shows intensive irregular roughness (melt fracture) at higher shear rates. The die swell becomes notable as degree of polymerization decreases...

Viscoelastic properties of propylene‐ethylene block copolymer melts (abstract)

Stress relaxation and creep behavior of 16 commercial propylene‐ethylene block copolymers was measured in a molten state. The relaxation moduli of samples with monomodal molecular weight distribution show plateau or shoulder at long times. The bimodality of molecular weight distribution in the latter samples is due to the ethylene‐propylene copolymer component with high molecular weight. The plateau at long times is more noticeable as the bimodality is stronger. The height and width of the plateau is independent of the ethylene content. A PP‐HDPE‐EFR blend which was made as a model material of propylene‐ethylene block copolymer does not show the plateau. The relaxation modulus at high temperature is always lower than that at low temperature, the time‐temperature superposition can be performed well in all the measured range, and the shift factor obeys the Arrhenius equation. It was assumed from the above experimental results that appearance of the plateau at long times originates mainly from the bimodality...